As noted, angle plates are currently used in the treatment of joint fractures. See U.S. Pat. No. 3,025,853 to MASON. Typically, an angle plate consists of a plate blade to be guided into the neck of the femur, which blade is permanently connected at a predetermined angle to a side plate to be attached to the femur shaft. The plate blade is driven into the bone at a predetermined angle, and the bone fragment near the joint is thereby gripped. After the plate blade has been completely driven into the neck of the femur, the side plate comes to rest laterally on the proximal femur shaft, where it is attached with a varying number of bone screws.
The advantages that can be achieved with this known angle plate are chiefly of a biomechanical type. Thanks to the broad support area, the blade permits optimum support in the cancellous area of the bone fragment near the joint.
The principal disadvantages of a conventional angle plate relate to its handling. Because of its single-unit construction, a great deal of attention must be paid to the orientation of the blade, as otherwise the longitudinal axis of the side plate will not accord with the longitudinal axis of the tubular bone. Another marginal condition is created by the anatomy of the bone to be set. The direction in which the plate blade is driven in, for example at the proximal femur head, from lateral to medial, must not cause the blade to break out sideways (toward the anterior or posterior) through the femur neck. If the three angular relations are defined by the surgeon, the exact point of entry of the plate blade must be determined. This point of entry is theoretically determined from the three aforesaid angle relations. In practice, there are often deliberate or accidental divergences. Accidental divergences are extremely difficult, if not impossible, to correct because they become visible only after the implantation of the plate blade has been completed. Since the dimension of the plate blade takes up almost the entire section of the femur neck when the plate blade is applied to the proximal femur (the most frequent application), a correction is possible to only a limited extent, and it leads to an unstable setting through the seat of the plate blade, which is now too large.
The implanting of such known angle plates thus requires an outstanding three-dimensional visualization ability on the part of the surgeon in the domain of anatomy, as well as careful, and thus time-consuming, preoperative planning.
Because of the difficulties that arise with known angle plates, many surgeons have changed to hip screws or condylar screws. With respect to handling during implantation, these screws offer major advantages, compared to the angle plate. One of the greatest advantages is the possibility of a "pre-exploration" of the screw position with the help of a guide wire. This guide wire is introduced, under x-ray monitoring, into the bone fragment to be set.
Since even these hip screws are driven into the sleeve of the side plate at a fixed angle, here again very precise work is required, via the use of a tracking device and a guide wire; as soon as the guide wire is in the best position in the bone, the hole for the hip screw is drilled over the positioned guide wire. After the hip screw has been driven in, the side plate can be pushed onto the screw shaft. A few known screw types have longitudinal grooves on the screw shaft, which must match with the corresponding lands on the sliding shaft (sleeve) of the side plate; this is not a problem, however, since if the matching is imperfect, the dynamic hip screw can be screwed in again or removed.
The disadvantage of the dynamic hip screw is the often unsatisfactory rotation stability and setting, and the large implant section. When one considers the loads that act on the implant, one recognizes that a hip screw is inferior to the angle plate. The same also applies to the setting of fractures in the area of the femur condylars.
An object of the present invention is to combine the biomechanical advantages of the angle plate with the simple operating technique of the hip screw.